The present invention relates to a high strength heat resisting steel of a high temperature steam turbine in a thermal power plant of ultra supercritical pressure and a steam turbine rotor which is made of the heat resisting steel
In recent years, with regard to thermal power generation plants, considerable attention has been paid to operating these plants under high temperature and high pressure with the goal of improving efficiency thereof, wherein it is intended to raise steam temperature of steam turbines up to 600.degree. C. from the highest steam temperature of 566.degree. C. at present, and finally up to 650.degree. C. In order to raise the steam temperature, a heat resisting material is required, which has a high temperature strength greater than conventional ferritic heat resisting steel Austenitic heat resisting alloys are hardly applied to such use since they are inferior in thermal fatigue strength due to a large thermal expansion coefficient and expensive production cost, while some of them are excellent in high temperature strength.
Thus, recently there have been proposed many new ferritic heat resisting steels which are improved in high temperature strength, for example, in JP-A-62-103345, JP-A-62-60845, JP-A-60-165360, JP-A-60-165359, JP-A-60-165358, JP-A-63-89644, JP-A-62-297436, JP-A-62-297435, JP-A-61-231139 and JP-A-61-69948 in all of which one of the present inventors participated. Among those ferritic heat resisting steels, it is believed that a steel disclosed in JP-A-62-103345 has the highest strength.
There have been also proposed other heat resisting steels in JP-A-57-207161 and JP-B2-57-25629, which are intended to be improved by the present invention. The present inventors further proposed another heat resisting steel as shown in JP-A-4-147948.
However, in order to achieve the ultimate steam temperature of 650.degree. C., those alloys mentioned above are not fully satisfactory, thus it has been desired to develop an available ferritic heat resisting steel having high strength at high temperature.
The heat resisting steel taught in JP-A-4-147948 is generally satisfactory. But, it has been found that, while the steel of JP' 948 has high strength at high temperature on the average, there is a large variance in high temperature strength and low temperature toughness thereof.
It is required to provide a rotor material which has 100,000 hours creep rupture strength of not less than 10 kgf/mm.sup.2 at 650.degree. C. in order to realize a thermal power plant of ultra supercritical pressure which is operated under the ultimate steam temperature of 650.degree. C. The rotor material is also required to be excellent in toughness property and brittleness resistance property in the view point of keeping safety against brittle fracture.